OYSTER BOTTOMS IN MATAGORDA BAY. 69 



DISTRIBUTION AND AVAILABILITY OF FOOD. 



In any given body of water in which the physical conditions of 

 precipitation, density, temperature, etc., are fairly constant there is a 

 more or less fixed limit to the amount of oyster food produced, very 

 much as there is limitation to the size of the crop that can under simi- 

 larly fixed conditions be grown on a given area of land. As, how- 

 ever, the diatoms and other organisms upon which the oyster feeds 

 are not permanently fixed to the bottom but suspended in the water, 

 it follows that their abundance fluctuates rather more than that of 

 land crops in general correspondence to the relative instability of 

 the water as compared with the soil. A high storm tide, for instance, 

 may carry away on its ebb large numbers of diatoms and materially 

 reduce the food value of the waters over the oyster beds. Such 

 phenomena are readily intelligible. There are others, however, con- 

 nected with the distribution and abundance of diatoms, which are 

 obscure as to their causes. It is a fact well known to students of 

 diatoms that not only their abundance in a given body of water but 

 the species themselves vary from year to year, and practical investi- 

 gators of the oyster beds observe the same fluctuations. In an ex- 

 perimental pond or claire at Lynnhaven, Va., where every effort has 

 been made to maintain practically uniform conditions, the rise and 

 fall of many species has been observed and it was not possible to 

 assign any cause for the changes. Oystermen and oyster growers 

 have indirectly remarked the same fluctuations, as their oysters one 

 year fatten and the next fail absolutely to get into condition for the 

 market, a phenomenon found everywhere on our coasts, but more 

 frequently occurring in some localities than in others. 



Undoubtedly there are for these irregularities physical and chemi- 

 cal causes which it may take years to elucidate, but for the failure of 

 the oysters to fatten in some localities there are sometimes causes 

 which it is by no means difficult to trace. Like land plants, diatoms 

 require for their growth certain soluble mineral salts, sunlight, and 

 air, all of which they obtain in the water, the medium in which they 

 live. The mineral salts, w T hich the land plant obtains through its 

 roots, bathe the diatoms on all sides, the water deriving them by solu- 

 tion of the materials of the bottom and from the leaching of the soils 

 of the drainage basins of the tributary streams. The former source 

 of supply must be fairly uniform year after year, and the latter, be- 

 ing dependent upon the precipitation, would appear, on the whole, to 

 conform to an average within certain limits, being less in dry years 

 and greater in wet ones, especially when freshets occur. In any given 

 body of water, therefore, with a fairly constant supply of salts in 

 solution there is a certain more or less definite limit beyond which the 

 production of diatoms can not proceed for lack of necessary nutri- 

 ment. To produce oysters of good size and quality a certain mini- 



